1. Field of the Invention
The present invention relates in general to an apparatus for detecting a pulse wave produced from an arterial vessel of a living body, and in particular to such an apparatus which is free from temperature effects.
2. Related Art Statement
An arterial pulse wave that is an oscillatory pressure wave produced from an arterial vessel in synchronization with heartbeat of a living body, provides information about not only intra-arterial blood pressure but also other conditions of the circulatory system of the living body. Thus, it is clinically needed to non-invasively detect a pulse wave produced from an arterial vessel, for measuring a blood pressure or making a diagnosis on pathology of the circulatory system of a patient.
There has been known a pulse wave detecting device for non-invasively detecting a pulse wave produced from an arterial vessel via the skin of a subject overlying the artery, by pressing the skin directly above the artery. The pulse wave detecting device of this type is disclosed by U.S. Pat. No. 4,423,738. This device includes a semiconductor substrate which is adapted to be pressed at one of opposite surfaces thereof (hereinafter referred to as the "press surface") against the skin of the subject. The press surface has a plurality of independent cavities formed therein, and the thus formed diaphragms of the semiconductor substrate have a thin wall. Each of the diaphragms is provided with a piezoresistor which serves for converting a strain produced in the diaphragm into an electric signal, namely, a pressure transmitted from the artery to the diaphragm. Meanwhile, each cavity is filled with a soft filler, such as a silicone rubber, which serves for transmitting a pulse wave from the skin of the subject to the corresponding diaphragm. This device is adapted to press the semiconductor substrate against the underlying artery via the skin and thereby obtain an electric signal representing a pulse wave produced from the artery.
However, the pulse wave detecting device of the above type suffers from a problem that an undesirable strain is produced in the diaphragm because of a temperature change, differently stated, a difference in thermal expansion between the semiconductor substrate and the soft filler. In the event that the difference in thermal expansion between the two members is large, an electric signal representing a pulse wave, generated by the device, may contain a temperature drift, therefore the pulse wave detected may not be accurate. If the output signal contains a temperature drift and the pulse wave represented by the output signal is not accurate, then the accuracy of measurement of blood pressure based on the pulse wave would not be satisfactory.